1. Field of the Invention
Aspects of the invention generally relate to apparatus and methods for heating substrates.
2. Background of the Related Art
In the fabrication of flat panel displays (FPD), thin film transistors (TFT), liquid crystal cells, metal interconnects and other features are formed by depositing and removing multiple layers of conducting, semiconducting and/or dielectric materials from a glass substrate. The various features formed are integrated into a system that collectively is used to create, for example, an active matrix display screen in which display states are electrically generated in individual pixels on an FPD. Processing techniques used to create an FPD include plasma-enhanced chemical vapor deposition (PECVD), physical vapor deposition (PVD), etching, and the like. Plasma processing is particularly well suited for the production of flat panel displays because of the relatively low processing temperatures employed during film deposition and because of the good film quality which results from plasma processes.
During FPD processing, uniform and controlled heat processing of films formed on a substrate is often critical for the FPD to function properly. The heating temperature required varies depending on the type of film being processed, and process being performed. For example, one exemplary type of film used in the construction of FPDs is low temperature poly silicon (LTPS). Part of the LTPS film processing requires the LTPS film be heated up to about 600° C. to remove hydrogen from the film, whereas a similar heat treatment for an amorphous silicon (α-Si) film requires a substantially lower temperature of up to about 450° C.
Generally, a film heating process is highly temperature sensitive as temperature non-uniformity may cause insufficient removal of unwanted contaminates, resulting in peeling and ablation of the film. To compensate for temperature non-uniformity, heating process time may be extended. Unfortunately, extending heating process time increases production cost, and may result in unusable films if the process is not completed (e.g., if heating process time is insufficiently extended).
Conventional heating chambers provide heat processing by heating one or more substrates through a combination of gas conduction and heat radiation. Unfortunately, chamber walls and other internal chamber components provide heat conduction paths within a chamber, resulting in conductive heat losses. The conductive heat losses create a constantly fluctuating substrate-heating environment. As the temperatures are increased, conductive heat losses become more pronounced, exacerbating the heat non-uniformity within the substrate-heating environment. Moreover, conventional heating chambers are often very large (to accommodate the substrate perimeter), further exacerbating the heating non-uniformity issues by increasing the area and volume to be heated. For example, to meet the demand for larger computer displays, monitors, flat-screen televisions, and the like, larger substrates must be employed. A typical FPD substrate may be 730 mm×920 mm or larger.
To compensate for larger substrates, larger chamber volumes, and the subsequent increase in heat losses, more heating elements may be used, thereby increasing the cost of the equipment, energy usage, and temperature non-uniformity. As temperatures increase, copper heating elements are often employed to offset energy costs and provide efficient heating. Copper heaters are generally more energy efficient than other types of heating elements. Unfortunately, as the temperatures are increased, copper atoms from the copper heaters often escape into the heating chamber and contaminate the film. Thus, traditional heating chambers and heating processes do not provide acceptably uniform and contaminant-free substrate heating for an efficient and cost effective substrate heating process.
Therefore, there is a need for methods and apparatus for uniformly heat processing a plurality of substrates.